Telescopes

Telescopes do two jobs: gather more light than the eye, and resolve finer detail. Refractors use a lens; reflectors use a mirror. All modern research telescopes are reflectors because large lenses suffer chromatic aberration, absorb light, and sag under their own weight — the largest refractor ever built (Yerkes, 40-inch, 1897) is essentially the physical limit.

Light-gathering power scales with aperture area (πd²/4), so a 4 m mirror collects 16× more light than a 1 m mirror. Angular resolution (the smallest detail distinguishable, measured in arcseconds) also improves with diameter — the diffraction limit is inversely proportional to aperture. Smaller arcsecond numbers mean finer resolution: a 2″ image beats a 5″ image.

Ground-based optical telescopes are limited to ~1″ resolution by atmospheric seeing. Solutions: build on mountaintops (Mauna Kea, Atacama), put them in space (Hubble, JWST), or use adaptive optics that flex the mirror in real time to cancel atmospheric distortion. Light is detected with CCDs — silicon chips that count photons per pixel.

Radio astronomy observes much longer wavelengths. Advantages: works 24/7, sees through clouds and interstellar dust, and reveals phenomena invisible at optical wavelengths (cold gas, pulsars, AGN jets). Disadvantage: long wavelengths give poor resolution for any practical dish size. Interferometry combines widely separated dishes (VLA, ALMA, FAST) to act as a single telescope kilometers across, matching optical resolution.

Other astronomies cover infrared, ultraviolet, X-ray, and gamma-ray bands. UV, X-rays, and gamma rays are blocked by the atmosphere and require space platforms. The same object can look completely different at each wavelength because different physical processes (cool dust, hot gas, energetic plasma) dominate in each band — multi-wavelength observation gives the fullest picture.